Field of the Invention
The present invention relates to a position output unit that outputs position information about a movable member to an external device, and a lens apparatus and lens system having such a position output unit.
Description of the Related Art
Lens apparatuses have movable optical members relating to zoom, focus, and iris, which are moved electrically or manually to produce optical changes, thereby enabling desired image shooting by cameras on which the lens apparatuses are mounted.
Virtual systems that combining an image of a real scene and a computer graphics image associated with the real scene image are widely used.
In the actual use of such a virtual system, position signals about the zoom, focus, and iris are fed to the virtual system. Then, the virtual system generates a computer graphics image that is matched in size, focus position, and brightness with a real scene image by using a computer in the system on the basis of the position information fed to the system. Thus, image combining which the viewer does not feel unnatural can be achieved even when the zoom, focus, and iris are changed in real time.
Japanese Patent Application Laid-Open No. 2000-106650 and Japanese Patent Application Laid-Open No. 2006-30656 disclose lens apparatuses that can be used in the above-described virtual system. The lens apparatuses disclosed in these patent literatures can output a digital pulse train signal generated by a rotary encoder linked with a movable optical member to the external virtual system.
The virtual system has a counter that counts the cumulative count value for pulse count number) of the digital pulse train signal output from the lens apparatus. The virtual system performs calibrating operation for initialing the counter as described in Japanese Patent Application Laid-Open No. 2000-106650, after which the position of the movable optical member can be determined from the pulse count value.
Thus, the virtual system can generate a computer graphics image matched in size, focus position, and brightness with a real scene image by using a computer in the system on the basis of the position information about the movable optical member.
The virtual system has its own maximum pulse count value, which is necessitated by its design constraints.
The number of pulses in a digital pulse train signal corresponding to the entire movable range of a movable optical member of a lens apparatus (i.e. the total pulse number over the movable range) varies depending on the resolution of position detection of a position detector and the movable range of the movable optical member. Therefore, in the case of a lens apparatus having a long movable range of a movable optical member or a lens apparatus equipped with a position detector having high resolution in position detection, the total pulse number over the movable range may exceed the maximum pulse count value.
As described above, a lens apparatus of which the total pulse number over the movable range exceeds the maximum pulse count value of a virtual system encounters the problem that the virtual system cannot determine the position of a movable optical member correctly.
The prior arts disclosed in Japanese Patent Application Laid-Open No. 2000-106650 and Japanese Patent Application Laid-Open No. 2006-30656 teach nothing about the total pulse number over the movable range with a lens apparatus.